Visit Barnes & Noble (or Books-a-Million or whichever one hasn't closed yet). Browse the aisles. When you find a rack—crime, comedy, history, etc.—where you feel your work best fits, that's your genre. At this point in the process (I assume you're at the querying stage), the genre you choose serves only to help the agent frame her take on your story when she reads your query; you want to be close so as not to mislead her. If you sell your book, your publisher's marketing department will decide the genre in which they're going promote it. What you said in your query letter will be long forgotten, usually along with your working title.